13C NMR spin–lattice relaxation times T1 are used to investigate the effect of low molecular weight diluents, including N,N-dimethylformamide, N-methylformamide, propylene carbonate, γ-butyrolactone, triglyme and tetraglyme, on the local polymer segmental motion in polyether–urethane networks. In all cases, an increase in the local mobility is deduced from the increasing T1 measurements consistent with a decreasing glass transition temperature. The extent of plasticization, however, is dependent on the nature of the small molecules. Those molecules which can either form strong polymer-diluent interactions (for example through dipolar interactions) or are themselves rigid, give the least enhancement of polymer mobility and the greatest deviation from the Fox equation for Tg. In the presence of alkali metal salts, N,N-dimethylformamide and propylene carbonate are shown to have opposite effects on the local polymer motion, as seen from the T1 measurements. In both cases, addition of the plasticizers increases the 13C T1 relaxation times for the plasticizer. However, propylene carbonate decreases the polymer 13C T1 whilst N,N-dimethylformamide results in the expected increase in polymer 13C T1.

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